1. Field of the Invention
The present invention relates to a carrier containing core material particles having magnetism and a coating layer on the surfaces of the core material particles, and also relates to a developer containing the carrier, an electrophotographic developing method and an image forming method using the developer.
2. Description of the Related Art
A developing process of electrophotography is classified into a one-component developing process using a one-component developer containing a toner, and a two-component developing process using a two-component developer containing glass beads, a magnetic carrier, or a coated carrier in which the surface of the glass beads or magnetic carrier are coated with a resin, and a toner.
The charge characteristics is more stable in the two-component developing process than in the one-component developing process, because the carrier has a larger frictionally charged area. Thus, the two-component developing process is advantageous in maintaining high-quality image over a long-period of time and has a high-ability of supplying a toner to developed areas. Thus, the two-component developing process is frequently used particularly in high-speed machines.
In an electrophotographic system employing a digital method in which a latent electrostatic image is formed on a photoconductor using a laser beam or the like, and the latent electrostatic image is formed into a visible image, the two-component developing method utilizing the above noted characteristics is also widely employed.
In recent years, to respond to increases in resolution, enhancements in high-light reproducibility of image, improvements in image granularity (roughness), and colorization, the minimum unit (one dot) of latent electrostatic image has been minimized, and image density growth has been improved. Especially, developments of image developing systems capable of developing these latent electrostatic images (dots) with fidelity have become extremely important, and there have been various proposals from both sides of developing process conditions and a developer (toner and carrier).
From the viewpoint of developing process, making developing gap closely contacted, making a thin layer for a photoconductor, and making smaller diameter of a writing beam diameter, etc. are effective, however, these solutions still leave problems in terms of high-cost and reliability.
From the viewpoint of a developer, making a smaller particle diameter of toner and making a smaller particle diameter of carrier have been studied, and there have been various proposals on use of a carrier having a small particle diameter.
For example, Japanese Patent Application Laid-Open (JP-A) No. 58-144839 proposes a magnetic carrier containing ferrite particles having a spinel structure and an average particle diameter of 30 μm or less, however, the proposed carrier is not coated with a resin and is used under low-electric field, and is disadvantages in that it is poor developing ability, and the operating life is short.
Japanese Patent (JP-B) No. 3029180 proposes an electrophotographic carrier containing carrier particles having an average particle diameter (D50) of 15 μm to 45 μm at a ratio of 50%, in which the carrier particles having a particle diameter of less than 22 μm at a ratio of 1% to 20%, carrier particles having a particle diameter of less than 16 μm at a ratio of 3% or less, carrier particles having a particle diameter of 62 μm or more at a ratio of 2% to 15%, and carrier particles having a particle diameter of 88 μm or more at a ratio of 2% or less, and the specific surface area S1 of the carrier determined by air permeability method and the specific surface area S2 of the carrier calculated by the following equation: S2=(6/ρ·D50)×104 (ρ represents a specific gravity of carrier) satisfy the formula 1.2≦S1/S2≦2.0.
When any of these above-noted carriers having smaller particle diameters is used, there are the following advantages:
(1) it is possible to give a sufficient frictional charge to individual toner particles because the carrier has a large surface area per unit volume, and the low-charge toner and oppositely-charged toner less occur. As a result, background smear hardly occurs, there is fewer amounts of toner dust and image blur in the areas around dots, and the use of the carrier makes it possible to obtain excellent dot reproducibility.
(2) it is possible to make the average charge amount of toner lowered because the carrier has a large surface area per unit volume and rarely cause background smear, and sufficient image densities can be obtained. Thus, the carrier having small particle diameters enables reducing troubles at the time of using a toner having small particle diameters, and is effective particularly in deriving advantages of use of a toner having small particle diameters.
(3) a carrier having a small particle diameter is capable of forming dense magnetic brush. Since the magnetic brush has excellent flowability, magnetic brush trails are hardly left on image surfaces.
However, the each of the proposed carriers having smaller particle diameters as described in JP-B No. 3029180 has disadvantages in that carrier adhesion easily occurs, and it is difficult to put them into practical use because the carrier adhesion causes occurrences of photoconductor flaws and fixing roller flaws.
In particular, when a carrier having a weight average particle diameter of less than 45 μm is used, the carrier surface smoothness is drastically improved, and a high quality image can be obtained, however, there are problems that carrier adhesion occurs very easily, and a high-quality image cannot be maintained over a long period of time.
JP-A No. 11-38752 discloses the description regarding a time constant. However, the time constant is measured under a contact state, and electric resistance R in the time constant is correlated to a static resistance Log R which is measured using a conventional cell. Thus, JP-A No. 11-38752 discloses an invention relating to a time constant of a developer containing a carrier and a toner, not to a time constant of a carrier.
Recently, there is a trend that the carrier diameter is made smaller for high image quality and high reliability. Because the surface area per unit weight is large (large specific surface area) in the carrier particles having small diameters, the carrier particles having small diameters do not easily release charge, compared to carrier particles having large diameters. Particularly, carrier adhesion caused by counter-charge, i.e. reverse-charge has been a big problem for the carrier having a small diameter.
The force Fc of causing carrier adhesion is associated with developing potential, background potential, centrifugal force applied on carrier, carrier resistance, and charge amount of a developer.
Thus, to prevent occurrences of carrier adhesion, it is effective to set various parameters such that the force Fc of causing carrier adhesion can be reduced. However, as it stands, it is difficult to drastically change the parameters because the force closely relates to developing ability, background smear, toner scattering, and the like.